1. Field of the Invention
This invention relates generally to methods of manufacturing integrated circuits in which particle beam projection systems expose a substrate through a mask. More particularly, the invention relates to an exposure mask having pseudo-random registration marks and the method of aligning a particle beam projection lithography system to achieve rapid and reliable registration of the mask and the substrate prior to exposure of the substrate by a beam of particles.
2. Description of Related Art
The tendency of integrated circuits in semiconductor technology is toward ever-decreasing structure dimensions in order to increase the density of the circuits and their switching speed. Photolithography, which is still used today in the majority of cases, is approaching the limits dictated by the physical resolution of optical systems. Structures having minimum feature sizes of less than 0.25 .mu.m in width cannot be made with optical systems. The most promising methods for the production of such fine structures are particle beam processes. The following specification refers specifically to electron beam systems but the pseudo-random registration mark system described herein can also be applied analogously to other charged particle beam projection systems, such as ion beam systems.
Electron beam systems have a number of advantages: the resolution of the patterns made with them is not limited by diffraction effects; they can be made with high intensity and they are deflectable with relative ease and high precision.
Many electron beam systems operate in accordance with the raster or scanning principle. The electron beam is used as a very fine "pencil" with which the pattern to be exposed is directly written onto a semiconductor substrate coated with an electron beam sensitive layer. The pattern to be produced is provided in the storage of a computer controlling the deflection of the electron beam. The high flexibility of this type of pattern generation, however, involves a high amount of writing time. The throughput of exposed wafers in industrial production is therefore low.
In the production of circuits and circuit chips (chips) having a plurality of repeatedly appearing circuit elements, e.g. memory chips, the flexibility of the raster method is of secondary importance. On the other hand, costs can only be reduced by high chip throughput. Electron beam projection methods utilizing a mask and operating analogously to optical photolithographic methods offer this high throughput since their larger pattern areas are imaged on the substrate by means of the mask through electron radiation. Such systems are known as described in the publications by H. Koops et al., Optik 28, 5, 1968/1969, pp. 518-531; T. W. O'Keeffe, Solid State Electronics, Pergamon Press, 1969, Vol. 12, pp. 841-848; M. B. Heritage, "Electron Projection Micro Fabrication System", Journal of Vacuum Science Technology, Vol. 12, 1975, pp. 1135-1138 and U.S. Pat. No. 4,169,230, U.S. Pat. No. 4,334,156 and U.S. Pat. No. 4,370,554, all assigned to the assignee of the present application.
The problem of increased production of highly integrated monolithic circuits, however, involves not only the possible resolution through the exposure method used but also the precision of the mutual alignment of mask and semiconductor substrate in each exposure step required during a manufacturing process. For achieving good overlapping (so-called "overlay"), registration has to be very precise. Registration is the detection of structures existing on the wafer prior to exposure, and the alignment of the pattern to be imaged (mask) relative to the existing structure.
Patterns for alignment (alignment marks) in electron beam processes are either markings of a material differing from that of the semiconductor substrate, and/or areas of a particular geometric design, e.g. edges. The impinging electron beam backscatters, producing a return signal which can be measured and utilized to determine when the mask is aligned with the substrate.
The quality of the return signal, and, therefore, the reliability and rapidity with which correct alignment can be achieved, is dependent in part upon the physical layout of the alignment marks used in the mask and substrate. One important characteristic is the transmission ratio of the alignment marks used. The ratio in the alignment marks of open areas (allowing the transmission of electrons) to the entire area of the mark affects overall signal level and the resulting signal to noise ratio. A high current density resulting from a high transmission ratio is desirable.
A second important characteristic relates to how easily the correct alignment can be detected. As the alignment marks on the substrate and mask are moved from an initially misaligned position towards an aligned position, the return signal should increase to a maximum level which occurs at perfect registration. As the correct alignment position is passed, the return signal should decrease.
The correlation function of the alignment mark on the mask with the alignment mark on the substrate controls this characteristic. The alignment marks should have a correlation function that produces a relatively large capture distance and a single easily identifiable peak in the return signal. The large capture distance allows the substrate and mask to start with a relatively large initial misalignment. The easily identifiable alignment peak requirement means that there are few or no false peaks that might lead to a false indication of alignment.
One prior art set of alignment marks is a series of evenly spaced parallel bars with the openings between bars having the same width as the bars. Such alignment marks are described in R. C. Farrow, et al., "Marks for Alignment and Registration in Projection Beam Electron Lithography", Journal of Vacuum Science Technology, B 11(6) November/December 1993, pp. 2175-2178. This type of alignment mark has a transmission ratio of 50% which provides a good return signal. However, the repeating bars result in a small capture distance and multiple false peaks in the correlation function that may lead to misalignment.
The distance over which capture occurs properly in the type of alignment system using repeating bars is slightly less than the distance between adjacent bars in the marks. If the substrate and mask are initially misaligned by too much, the opening between two bars in the mask will initially be located over a different opening in the mark on the substrate. As a result, false capture will occur in which the first bar in the mask is positioned over the second bar in the substrate instead of over the corresponding first bar in the substrate and the alignment will be off by the distance between bars in the mark. This problem results from the regular spacing of the bars and spaces in the alignment mark when an opening in the mark on the substrate aligns with a different opening on the mask.
For achieving a high signal to noise ratio the electron beam impinging at the alignment mark should have a high current density. This is relatively easy to achieve in raster processes. In mask exposure, however, the projection methods operate with an expanded electron beam of a relatively low current density. The secondary electrons released by this beam at the conventional alignment marks provide very low registration signals with a high noise factor.
Consequently, when alignment mark patterns are used that have low transmission ratios, it is more difficult to detect the return signal and achieve proper registration. On the other hand, a large capture distance and the absence of false peaks in the correlation function are also important to improving alignment reliability. Prior art alignment systems that have attempted to improve these characteristics of the correlation function have done so by changing the shape of the alignment marks in ways that result in a significant decrease in the transmission ratio.
One possible improvement in the current density and overall signal level consists in the multiple deflection of the beam over the mark with subsequent electronic integration for improving the signal to noise ratio, but this process is time-consuming.
Another suggestion involves switching the electron beam from one exposure mode (with low current density) to a registration mode with high current density (see the article by Heritage). However, the switching of the beam path is difficult to accomplish since the two beam paths cannot always be reproduced.
According to another suggestion (8th International Conference on Electron and Ion Beam Science and Technology, May 1978, page 984, Frosien et al.), switching is abandoned and mechanical diaphragms are pivoted into the beam path instead. However, such mechanical adjustments are not realizable within very short periods.
For automatic as well as manual registration, a method for quickly reaching a registered position should be provided. For this purpose the relative position of the objects at any moment and the direction and shift of the objects to reach the registered position must be known. In German Auslegeschrift 20 46 332, an alignment pattern is described for a photoelectric device, where the spacings between two respective mask openings are not constant. The spacing of the openings corresponds to an arithmetic series. For two-dimensional alignment, two rows of such mask openings arranged vertically to each other are provided. This arrangement has the disadvantage of a low transparency (ratio between the surface of the openings to the overall surface) so that if subjected to electron beam exposure systems the signal to noise ratio would be low. This arrangement also requires two detectors.
U.S. Pat. No. 4,370,554 describes a particle beam lithography system in which the alignment marks are spaced bars similar to the parallel bars described above, but in which the distances between successive bars are set such that no distance can be represented by the sum of smaller distances. This improves the correlation function, but at the expense of decreasing the transmission ratio.
It is therefore the object of the present invention to provide an exposure mask for a particle beam projection system which permits quick, precise and reliable automatic registration of the mask relative to the substrate.
It is another object of the present invention to provide an exposure mask for a particle beam projection system utilizing alignment marks which have a good transmission ratio to provide a good signal to noise ratio.
A further object of the invention is to provide an exposure mask for a particle beam projection system utilizing alignment marks which provide a large capture distance and a single easily identifiable peak in the return signal.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.